Field of the Invention
This invention relates generally to a LiDAR sensor and, more particularly, to a LiDAR sensor for automotive applications, where the sensor employs a plurality of vertical cavity surface emitting laser (VCSEL) arrays each including a separate lens.
Discussion of the Related Art
Modern vehicles sometimes include various active safety and control systems, such as collision avoidance systems, adaptive cruise control systems, lane keeping systems, lane centering systems, etc., where vehicle technology is moving towards semi-autonomous and fully autonomous driven vehicles. For example, collision avoidance systems are known in the art that provide automatic vehicle control, such as braking, if a potential or imminent collision with another vehicle or object is detected, and also may provide a warning to allow the driver to take corrective measures to prevent the collision. Also, adaptive cruise control systems are known that employ a forward looking sensor that provides automatic speed control and/or braking if the subject vehicle is approaching another vehicle. The object detection sensors for these types of systems may use any of a number of technologies, such as short range radar, long range radar, cameras with image processing, laser or LiDAR, ultrasound, etc. The object detection sensors detect vehicles and other objects in the path of a subject vehicle, and the application software uses the object detection information to provide warnings or take actions as appropriate.
LiDAR sensors are sometimes employed on vehicles to detect objects around the vehicle and provide a range to and orientation of those objects using reflections from the objects providing multiple scan points that combine as a point cluster range map, where a separate scan point is provided for every ½° or less across the field-of-view (FOV) of the sensor. Therefore, if a target vehicle or other object is detected in front of the subject vehicle, there may be multiple scan points that are returned that identify the distance of the target vehicle from the subject vehicle. By providing a cluster of scan return points, objects having various and arbitrary shapes, such as trucks, trailers, bicycle, pedestrian, guard rail, etc., can be more readily detected, where the bigger and/or closer the object to the subject vehicle the more scan points are provided.
Most known LiDAR sensors employ a single laser and a fast rotating mirror to produce a three-dimensional point cloud of reflections or returns surrounding the vehicle. As the mirror rotates, the laser emits pulses of light and the sensor measures the time that it takes the light pulse to be reflected and returned from objects in its FOV to determine the distance of the objects, known in the art as time-of-flight calculations. By pulsing the laser very quickly, a three-dimensional image of objects in the FOV of the sensor can be generated. Multiple sensors can be provided and the images therefrom can be correlated to generate a three-dimensional image of objects surrounding the vehicle. Other known LiDAR sensors rotate the entire laser system instead of just the mirror to provide signal scanning, but such systems typically suffer from being bulky and typically lack robustness.
The mirror referred to above is a relatively large and dominate part of the LiDAR sensor, and thus is responsible for much of the size and mass of the sensor, which also increases the cost of the sensor. Moreover, the mirror causes poor sensor robustness. Further, the single axis of the mirror means that the LiDAR sensor does not actually collect a real three-dimensional point cloud of returns, but a point cloud more similar to a two-dimensional distance over a single line, or multiple lines if more than one laser is employed. Particularly, because a single laser beam is scanned using a rotating mirror, the reflected beams do not provide a FOV in a direction vertical to the direction of the rotation of the mirror. In order to provide returns from those directions, additional lasers need to be provided, which provides limitations in design and increases costs. In order to fulfill the need for quick data updates, the mirror must rotate very fast, which only allows the measurement algorithms to be time-of-flight, which is also very costly as a result of the need for fast electronics. Also, the corresponding motor required to rotate the mirror adds significant size and weight to the LiDAR sensor. Further, because the mirror is large and bulky, it has a tendency to easily get out of alignment with the laser even in response to small disturbances on the vehicle. Also, the motor requires significant power and because the mirror rotates, there is a significant increase in the likelihood that the LiDAR sensor may fail as a result of mechanical wear. Thus, current LiDAR sensor designs are generally too costly to be implemented in mass produced vehicles.
Additionally, current LiDAR sensor designs do not allow for the signal returns from one sensor to another sensor, and as such with multiple vehicles operating in the same relative space, cross talk between sensors is an issue.
Another known LiDAR sensor is referred to as a flash LiDAR sensor that employs a single powerful laser that illuminates the entire sensor FOV. The sensor includes a fine array of special detectors that provide time-of-flight (TOF) range calculations to determine the distance to objects. These sensors tend to require a powerful and costly laser, a bulky and expensive imaging lens that has high resolution because the pixels are very small, the FOV is large and a significant amount of light needs to be collected, and a custom sensor array.
It is known in the art to provide a LiDAR sensor that employs a two-dimensional array of lasers and associated lens so as to create an opto-electronic scanning technique with no moving parts. The two-dimensional array of lasers can be a VCSEL array, known in the art, that is a semiconductor type laser, where each laser point source is fabricated on a wafer to the desired size. Each laser in a VCSEL array is electrically controlled so that the selected laser in the array can be switched on and off as desired. Therefore, by selectively turning the lasers on and off, the laser beam from the array is scanned to allow a three-dimensional return point cloud because the lasers are not aligned in a single line. However, this known LiDAR sensor is limited in its FOV because for larger FOVs the lens design that would be required to produce high spatial resolution would be very difficult to achieve, especially at low cost. Even known fish eye type lenses having wide FOVs are still limited in their FOV and provide poor resolution at their edges. Further, as the size of the VCSEL array increases to accommodate a wider FOV, the cost for providing such a large VCSEL array increases significantly. Also, the combination of large FOV and well collimated beams required for resolution and eye safety is difficult to obtain at low cost.